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The present invention relates to a GSTN (General Switched Telephone Network) modem. Specifically, the invention relates to methods and systems using an adaptive digital filter for echo cancellation.
Echo Cancellation is a technique typically used for improving full duplex communications on a two-wire subscriber loop. In this context, an echo canceler is typically an adaptive filter for estimating an impulse response of an echo channel. The echo estimate is then subtracted from the received signal in order to remove the echo signal component.
Typically, a two-wire, full-duplex subscriber loop connects a subscriber to the GSTN via a central office. Hybrids are used at the subscriber end of the loop and at the central office to convert the two-wire subscriber loop to a four-wire full-duplex circuit. At the subscriber end of the loop, conventional telephones and modems employ hybrids to convert the two-wire subscriber loop to a four-wire full duplex circuit. The two-wire subscriber loop then may be used for the transmission of data to and from a modem.
For high speed applications, modems are required to tolerate a vast number of diverse channel and echo impairments associated with the communication channel. A typical echo canceler reduces the echo signal by employing an adaptive digital transversal filter to model the impulse response of the unknown echo channel. A transversal filter, which is also known as a tapped delay line filter, is a type of finite impulse response (FIR) filter. Most common echo cancelers use a transversal filter because the optimal coefficient values for convergence of the filter are well known. Echo cancellation using a FIR filter is achieved by updating the echo impulse response coefficients used in the transversal filter to track the characteristics of the unknown echo channel. However, a significant disadvantage is that as the echo path delay increases, the number of taps required of the transversal filter increases proportionally.
Conventional full duplex modems are typically implemented using dedicated DSP (Digital Signal Processing) hardware. Most DSP-based modems utilize one or more finite impulse response (FIR) adaptive filters to perform echo cancellation. Each filter is required to have a particular number of taps or delay units based on the maximum echo path delay. Generally, the larger the number of taps, the better the filter performs its filtering function. However, an associated disadvantage of such a long delay FIR filter is increased processing time. To meet the required level of echo cancellation, the transversal filter impulse response must span over all symbol intervals, which causes increased convergence time. In addition, there is an increased cost associated with the increased computational complexity of implementing a transversal filter having a large number of taps.
As an alternative to the FIR filter approach, one might consider implementing the echo canceler as an infinite impulse response (IIR) filter. The primary advantage of an IIR filter is that a long delayed echo can be estimated using a relatively small number of filter taps. A disadvantage of using an IIR filter, however, is that by their very nature these filters are potentially unstable.
An article entitled, xe2x80x9cReduced Complexity Echo Cancellation Using Orthonormal Functions,xe2x80x9d IEEE Transactions on Circuits and Systems, Vol. 38, January 1991, Gordon W. Davidson and David D. Folconer, the contents of which are incorporated herein by reference, demonstrates a two-stage echo cancellation process. A disadvantage of using the echo canceler proposed by Davidson/Falconer is that the two-stage process requires the received signal to be modified twice before echo cancellation is complete.
It would therefore be desirable to have an improved echo cancellation method and apparatus.
An object of the invention is to improve the performance of an echo canceler. An apparatus and method is described for canceling transmitted echo signals from a receive signal by generating an improved echo estimation. By using a Laguerre filtering technique in association with a FIR (transversal) filter, the size of the FIR filler is reduced while providing the performance of a larger more complex FIR filter.
In accordance with a first aspect of the invention, a method is provided for canceling echo from a received signal. The method begins by storing a series of transmitted symbols in a buffer. The buffer may be partitioned into a head portion and a tail portion, where the head portion relates to the head of the echo impulse response and the tail portion relates to the tail of the echo impulse response. A Laguerre filter is applied to symbols in the tail portion of the buffer. The symbols in the tail portion are replaced with the filtered symbols. Next, the method includes generating an echo estimation signal based on the symbols in the buffer.
In accordance with a second aspect of the invention, an improved echo canceler is provided. The echo canceler includes a transmit symbol buffer. A modulator is coupled to the transmit symbol buffer. A Laguerre filter is coupled to the modulator. A demodulator is coupled between the Laguerre filter and the buffer. An echo estimator is coupled to the buffer.